Ampleon 250w pallet

  • Hi,


    I've preferred to by the complete module, I had a lot of problem with temperature and bias but now look working well. Let me know how your pcb works when you finished it.


    Bye!

    Hi Marco,


    Do you have a schematic diagram for the BIAS controlling?
    I have a BPC2425M7X60 Ampleon pallet. That is a bit more complicated because of more stages but I think your solution is good for me too.


    Thanks,

    Tamas

  • Hi,


    the Ampleon 250w pallet has been a good challenge I had to fight a lot with bias circuit before understanding
    the real problem root cause.


    I had really to read carefully the data sheet and application note.


    First of all we have to think what is the pallet designed for. Not at all for DVB-S2 but for CW or CSW pulsed.

    We do not have to forget it is for ISM energy professional usage, continuous operations.


    If you have look on the Ampleon application
    note, the water cooling system is recommended, you know when an American designer is writing recommended it means
    we mast follow the recommendation. Not only the pallet requires water cooling but cold water cooling, yes 5 degree

    centigrade water cooling.


    The pallet data sheet says there is an internal bias compensation but I've not been able to find any circuit details.

    I'm in a good relationships with Ampleon people, I've opened several support tickets getting fast and detailed answers,

    but no way to get the bias details. I've then done pcb pallet reverse engineering finding the internal bias compensation

    it is just one 1500ohm ntc feeding the bias input current to ground. I had a look on the BPC2425M7X60 and I think
    it has also same ntc as 250w pallet, so using this 60w pallet you will run same problem I had.


    Here attached the ntc resistance/temperature table I've got from ampleon. Reading that table you see the ntc has
    around 1500ohm at 25 degree centigrade and the resistance drops dramatically to 600ohm at only 40 degree centigrade.


    I've chosen a really simple external bias circuit done by a regulator from 28v to 12, than a couple of resistors and a

    potentiometer generating a bias voltage from zero to 8V. The 250w pallet has 1,6v threshold.


    I've than applied the 28V and adjusted the bias to 500ma as suggested from ampleon tiket regarding DVB-S2 usage.


    I've than applied rf input and seen the output power was initially getting well and after a couple of second dropping

    dramatically.


    I've than measured the operating bias directly to the LDMOS gate discovering it was drooping from initial 1.7V to 0.5 volt.


    I've than thought the bias current was not enough.


    Even I know the LDMOS bias current is around one microampere, I've decided to add on transitory to my bias circuit to

    increase the current. I've than adjusted the bias current to 500ma and applied the rf in. Nothing changed same behavior

    as before good initial power and than dropped again.


    Luckily the pallet has another ntc, same as the bias compensation one called temperature sensor, measuring it resistance
    I've discovered just applying rf in it drops from 1500ohm to 600ohm in a couple of second.


    I do not want to bore you with all the tests I've done, but at the and I came to the conclusion I had two options, the first
    add cold water cooling to the pallet, the second take out internal pallet bias compensations.


    Due to the fact I do not have continuous operations needs for amateur DVB-S2 operation, I've taken out the ntc from
    the pallet and added an ohmmeter to the ntc temperature sense. Looking to the ampleon ntc table I check the ohmmeter

    not drooping less to 200 ohm corresponding to 60 degree centigrade. That is the maximum pallet temperature suggested
    by ampleon.


    Since than I'm using the pallet for DVB-S2 operation with 140w power and I've never seen the internal sense dropping

    less than 550ohm.


    The best would bee to add an alarm circuit to switch off the pa when the internal sense drop to 200 ohm.


    I'm now working in an ampleon 500w LDMOS pa I think I'll do the alarm for it.


    I hope I've my experience will help someone else working with ampleon pallet.


    Bye, ... have fun with bias and ntc.


    DE IZ4RYB

  • ...I've taken out the ntc from

    the pallet and added an ohmmeter to the ntc temperature sense. Looking to the ampleon ntc table I check the ohmmeter

    not drooping less to 200 ohm corresponding to 60 degree centigrade. ...

    thank you Marco for the detailed description.


    Do you think it is possible to adjust the NTC with parallel or series resistors, or to use another NTC?


    73 de Kurt, DJ0ABR

  • Hi,


    this kind of thermal bias NTC compensation is suitable only for CW operation with a bias current around 20ma, using water cooling.

    When the LDMOS is used around its maximum power the NTC reduce the bias current and the output power decreases leaving the
    LDMOS temperature under 60 degrees. We are using the LDMOS at half of the maximum power with around 800ma bias current.

    If we leave any kind of NTC thermal compensation we are not getting any power decrease with bias we will only reduce the bias
    current and the linearity creating DVBS distortions.

    The best DVBS thermal compensation suitable for DVBS operation, should be a microprocessor measuring with internal ADC the NTC
    resistance and regulating the input power with a digital step attenuator.

    We are not a SAT broadcasting TV so we do not need to have continuous operation, so I've just placed one ohmmeter connected to
    the internal pallet NTC sensor and I just check the resistance stays around 400 ohm during Tx.


    Bye!